TITLE: GCN CIRCULAR NUMBER: 14666 SUBJECT: GRB 130427A: Continued RATIR Optical and NIR Observations - Photometric Evidence for a New Component DATE: 13/05/16 01:57:40 GMT FROM: Alan M. Watson at Instituto de Astronomia UNAM Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UCSC), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and Harvey Moseley (GSFC) report: We have continued to monitor GRB 130427A with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir, obtaining homogenous photometry in griZYJH. We have photometry for every night except 2013 May 6. On most nights our photometric uncertainties in gri are about 2%. As we reported earlier in Watson et al. (GCN Circular 14606), the optical afterglow during the first day is well-fitted by a power law with a temporal index of -1. However, around T+1d there is a break, and the power law steepens. From T+2.5d to T+14.9d our gri photometry is well-fitted by a power law with a temporal index close to -1.5 plus a constant component consistent with the presumed SDSS host galaxy. However, our observations at T+15.9d, T+16.9d, and T+17.9d are systematically brighter than this fit. Adding a new component starting at T+15.5d with zero colors and constant magnitude significantly improves the fit (with a confidence level of better than 99.5%). The constant component has g = r = i = 24.53 ± 0.25. We do not mean to suggest that the new component actually has zero color or constant magnitude. However, at this moment our data cannot usefully constrain anything other than a characteristic brightness. Our data, model, and residuals are shown at https://dl.dropboxusercontent.com/u/528672/GCN/2013-05-16-GRB-130427A.pdf Assuming a distance modulus of 41.26, the new component corresponds to an absolute magnitude of -16.7 ± 0.25. If the new component is a Type 1c supernova, as suggested by de Ugarte Postigo et al. (GCN Circular 14646), we might expect the peak extinction-corrected absolute magnitude to be around -18 (Drout et al. 2011, ApJ, 741, 97). Thus, depending on the host galaxy extinction, we might be seeing this possible supernova at or just before its peak. We caution that the new component is currently about 2 magnitudes fainter than the afterglow component, which at 18.0 days is predicted to have i = 22.21 ± 0.04 and even fainter then the galaxy, which is predicted to have i = 21.23 ± 0.03. The relative brightnesses of the new component, the fading afterglow, and the host galaxy also have significant implications for unveiling the spectrum of the possible supernova. We further caution that from our data alone we cannot exclude the possibility that the new component might simply be a significant flattening of the late afterglow component. The largest residuals of our observations from the model (with or without the new component) are at the level of 0.05 magnitudes. We do not see the large variations reported by Trotter et al. (GCN Circular 14662). Further observations are planned. We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.